I think that mabey the reason (which would be a great plot twist for Halo 2) why the covenant found Earth is that remember the space probe Pioneer 10? Pioneer 10 (click here for more info) that "carries a famous gold plaque--an attempt by humans to communicate with other beings should the spacecraft be found by aliens during its journey through deep space."

we could have baisiclly screwed ourselves for the covenant finding Earth! if it is already 10 Billion miles out now from 1983 (only 30 year from the launch) think about 600 years from now!! it decribes how to get to Earth! Makes a good story to me!